We propose a combined basic and clinical investigation on the contribution of reactive oxygen species (ROS) to DNA damage, mutations and cancer. We hypothesize that an important source of the multiple mutations found in cancer cells is the extensive ROS-induced damage generated by endogenous processes and by numerous environmental agents. We have established new approaches for quantitating ROS-induced DNA damage and mutations. Firstly, we have evolved RNA molecules that bind with high affinity and specificity to the well characterized ROS-generated lesion in DNA, 8-oxo-deoxyguanosine. Secondly, we have established an assay for frameshift mutations in repetitive nucleotide (microsatellite) sequences, which we find to be preferentially damaged by ROS. We will use these approaches to study human cells exposed to agents that induce ROS and to conditions that alter proliferation, such as might occur during tumor growth and invasion. In addition, we will analyze breast and prostatic adenocarcinoma cells for microsatellite instability and for ROS-induced DNA damage. The RNA binders will allow us to study the heterogeneity of DNA damage in histological and cytological preparations of tumors. By the time a tumor is detected, it contains multiple mutations, some of which may be required for growth, invasion and metastasis. Delaying the rate of accumulation of mutations provides a new approach for the prevention of cancer deaths. Our goal is to determine if oxygen-induced DNA damage is a significant contributor to the multiple mutations that occur during tumorigenesis. If so, we might be able to diminish the levels of ROS in cells and thus slow the rate of tumor progression. Even a modest, 2-fold, decrease could increase the latent period before disease is manifested clinically and thus reduce cancer mortality.